The present invention relates to an improved endoprosthesis, and more specifically to an improved endoprosthesis made of a hollow, collapsible plastic that can be filled after installation. The invention also relates to a method for installing such a prosthesis in a manner that makes possible a significant reduction in trauma to the patient.
In addition, the invention relates to a tool which enables the installation of a hip prosthesis embodying the present invention by removal of the ball of the femur through a portal in the lateral cortex.
As a result of injuries, arthritis and other diseases, as well as degenerative processes that result from aging and/or the use of certain drugs, it often becomes necessary to surgically replace a ball-and-socket joint, i.e., the shoulder or hip, with a prosthesis. In the hip, the replacement of the head, or ball, of the hip with a metal endoprosthesis has become a relatively common surgical procedure. Often the acetabular cup is also replaced in a procedure commonly referred to as a "total hip replacement."
Not only is such a procedure often required in humans, but it is also sometimes required in animals such as dogs. However, owing to the expense of such a procedure, it is rarely performed on animals.
The installation of a conventional endoprosthesis requires that extensive muscle tissue be cut in order to expose the joint structure. In the case of a hip endoprosthesis, the hip itself must then be dislocated, the ball and neck removed, and marrow removed from the bone. The endoprosthesis has a long stem portion which is then installed in the cavity left by the removed marrow. The endoprosthesis is then cemented into place with a suitable cement such as methyl methacrylate.
A significant problem with the use of metal endoprostheses is their tendency to work loose and to become separated from the bone structure. While there are several causes for this, such loosening is unquestionably promoted by the difference in hardness between the endoprosthesis and the much softer surrounding bone structure. Breaking loose is also promoted by the inability of a metal endoprosthesis to conform to the interior of the femur. That is, the endoprosthesis must be shaped so that it can be slid into the bone, and this shape inevitably promotes breaking loose. Finally, the fact that the endoprosthesis is made of metal prevents the normal bending and compression of the femur in the area proximal to the stem.
Another problem with conventional endoprostheses is the high expense involved. Not only do surgical procedures tend to be lengthy and expensive, but the endoprostheses themselves are a significant expense. Such devices are expensive to manufacture, and a variety of sizes must be carried in inventory, since it is important to obtain as close a fit as possible for the individual patient.
Loosening is a particular problem in the case of a hip endoprosthesis, since that area of the body is subjected to frequent motion and is required to carry weight. When loosening of the endoprosthesis occurs, the correction of this problem usually requires a second major surgical procedure to remove the endoprosthesis and substitute one with a larger stem in order to fill the now enlarged interior of the femur. Such surgical procedures can be extremely traumatic, particularly in older patients, among whom the majority of hip replacements are performed.
Previous efforts at overcoming these problems have primarily been directed at attempts to improve the bonding between the endoprosthesis and the surrounding bone structure. For example, it is widely believed that a major source of loosening is that the curing of commonly used cements is an exothermic reaction which itself destroys some of the tissue on the interior of the femur. Such tissue destruction reduces the ability of the patient's tissue to grow and form a bond with the endoprosthesis. In an effort to correct this problem, "cementless" metal endoprostheses have been developed, which rely on a roughened surface into which the tissue can grow. Because time is required for such tissue growth, cementless endoprostheses greatly extend the healing time following surgery. Moreover, because it is made of metal, a cementless endoprosthesis will still tend to transmit shock to the bone, and, in many cases, will still tend to work loose.
Another problem with conventional metal endoprostheses is that the hard metal ball tends to destroy the relatively soft tissue of the joint socket. Thus, when an endoprosthesis is installed alone, it often becomes necessary to later replace the socket. As is well known in the art, the replacement of the socket itself can lead to an additional set of difficulties in that the replacement socket may tend to come loose.
Owing to the trauma involved in the surgery, and to the aging of the population, there is a growing number of people who need joint replacements, especially hip replacements, but cannot tolerate such surgery. Thus, there is a need for an endoprosthesis that can be installed with a method that involves significantly reduced trauma to the patient.